The basic functionality of a Global Positioning System (GPS) receiver is to calculate the latitude, longitude and altitude of the GPS receiver's location (i.e., the co-ordinates of the receiver) upon receiving a number of GPS signals from a network of GPS satellites that orbit the earth. The calculation of the co-ordinates of the GPS receiver typically begins by comparing the timing associated with a select number of received GPS signals. After the initial comparison of the received GPS signals, values for timing corrections associated with the select group of received GPS signals are established. The timing corrections are made in order to solve a three-dimensional geometric problem, which has as its solution the co-ordinates of the GPS receiver.
The received GPS signals are typically weak and thus easily interfered with by other radio transmissions in the same or adjacent frequency bands. Interference can be especially problematic when the GPS receiver is co-located with a communications device that includes a radio transmitter, such as a cellular telephone. The transmitted signal from the co-located communication device can overload (or saturate) the GPS receiver front-end designed to receive weak GPS signals. In such a situation no useful information can be extracted from the received GPS signals originating from the GPS satellites.
In select instances this problem may be overcome by filtering all of the received signals from the GPS antenna before down conversion of the respective transmission signal band by the GPS receiver front-end. Typically a low noise amplifier (LNA) is first used to amplify the signal before further filtering or mixing to another frequency. The result of adopting this approach is that the loss of all signal energies in the filter reduces the sensitivity of the GPS receiver permanently, irrespective of whether or not the co-located communications device is transmitting. This is undesirable as the GPS signals received from the satellites are weak and reducing the sensitivity of the GPS receiver further reduces the operability of the system. Additionally, the filter would also occupy space, add cost to the unit and would draw additional power.
The problem caused by the co-located communications device may be alternatively overcome by the use of a high linearity LNA. This would ensure that the LNA is capable of amplifying the GPS signal despite the presence of a large interfering locally generated transmission. The disadvantage of this solution is that such an LNA would consume additional power, which is not acceptable in a portable battery powered device such as a cellular telephone. A filter following the LNA would also be required to provide sufficient rejection of the interfering signal to prevent overload of the next stage of the receiver, typically a mixer. These additional performance requirements increase the size, power consumption and cost of the filter and make implementing a highly integrated receiver design without the additional filter difficult.
Given that it is not easily possible to remove the effect of the interfering transmission, it is important to achieve the maximum performance possible despite the interference. A method of achieving this that has been commonly used is to employ a ‘blanking’ signal, derived from the transmitter of the co-located communications device and active whenever that transmitter is switched on, which is used to suppress the operation of the GPS receiver during the transmission. The disadvantage of this is that such a signal is not always easily derived from the co-located transmitter. Even if such a signal can be derived from the co-located transmitter, the physical construction of the unit may preclude the connection of the signal into the GPS receiver. For example, the GPS receiver and the communications device, while co-located, may not be physically constructed as a single unit. Furthermore, there may be more than one communications device, such as a cellular telephone with additional functions such as a short-range radio link.
Under these circumstances, it would be advantageous if the GPS receiver can determine for itself the period during which a co-located transmitter is active and take such action as to mitigate as far as possible the loss of performance caused by the interfering transmission.